The use of platinum-group metal oxides as electrocatalytic coatings on titanium and other valve metal electrodes was first described in U.K. Patent Specification No. 1 147 442 which recognized the particularly advantageous properties of palladium oxide. Subsequently, U.K. Patent Specification No. 1 195 871 proposed coatings formed as a mixed-crystal or solid-solution of a valve-metal/platinum-group metal oxide, and such coatings in particular ruthenium-titanium oxide coatings have been very widely used on so-called dimensionally stable anodes in mercury, diaphragm and membrane cells for chlorine production. Example VII of the latter patent proposed a palladium-tantalum oxide coating for cathodic protection or hypochlorite preparation, but this coating has not met with success.
Many efforts have subsequently been made to provide electrodes with a palladium oxide based electrocatalyst, but without great success.
For example, Japanese Patent Application Open No. 51-56783 proposed a coating of 55-95 mol % PdO and 5-45 mol % RuO.sub.2, but these coatings have a very poor lifetime, and an attempt to remedy this was to provide an underlayer e.g. of RuO.sub.2.TiO.sub.2 (Japanese Patent Application Open No. 51-78787). Another suggestion, in Japanese Patent Application Open No. 51-116182, was a coating consisting of 3-65 mol % PdO, 3-20 mol % RuO.sub.2 and 20-90 mol % TiO.sub.2, but again poor results were encountered.
Further attempts to derive advantages from the properties of palladium oxide include:
a composite coating of palladium oxide with tin oxide and ruthenium oxide and possibly with titanium oxide in specified proportions (U.S. Pat. No. 4,061,558);
palladium oxide combined with tin, antimony and/or titanium oxide (Japanese Patent Application Open No. 52-58075);
an underlayer e.g. of platinum or RuO.sub.2 topcoated with palladium and tin oxides (Japanese Patent Application Open No. 52-68076);
palladium oxide with a small amount of ZrO.sub.2 or CeO.sub.2, possibly up to 20 mol % of the PdO being substituted by, e.g. RuO.sub.2 (Japanese Patent Application Open No. 53-33983);
a partially oxidized platinum-palladium alloy (U.K. Patent Specification No. 1 549 119);
palladium oxide and platinum produced by thermal decomposition (Japanese Patent Application Open No. 52-86193);
pre-formed palladium oxide dispersed in platinum produced by thermal decomposition (Japanese Patent Application Open Nos. 54-43879 and 54-77286);
a sub-layer of platinum coated with PdO, CeO.sub.2 and TiO.sub.2 (Japanese Patent Publication Open No. 54-102290); and
a coating of PdO-Pt-SnO.sub.2 (Japanese Patent Publication Open No. 55-97486).
These publications illustrate the efforts made to employ palladium oxide on account of its good technical properties, in particular its low chlorine evolution potential and high oxygen evolution potential, and its moderate cost. However, none of the expedients or combinations proposed to date has effectively realized the potential advantages of palladium oxide because of the inherent difficulties involved and in particular its poor stability.